In many industrial plants hot wash water, after use, is sent to a water drain from a batch tank and cold water is then drawn from a main to refill that tank or another tank. The cold water must then be heated by steam coils or other energy means. This process is often repeated with each rinse cycle many times during the course of a year. Since hot water is what is desired, and energy is used to heat the cold water, it would be desirable to preheat the incoming water from the main by recouping heat from the hot waste water in order to reduce energy costs. Prior art simple process-heat exchanges are not suitable for such batch-process applications, because of intermittent flows and variable temperatures encountered.
Thermal reservoirs are used when hot waste is discharged but cold feed at a balanced flow rate is not flowing at the same time. The thermal reservoir in this instance serves to store heat from the hot stream and at a later time when energy is needed transfers heat into the cold stream. There are two main types of thermal reservoirs, the open system and the closed system. The open reservoir system stores heat in the working fluid. The closed reservoir system receives heat and stores it in an intermediate thermal medium, such as rocks or pressurized water, before transferring it to the stream to be heated.
Where the temperature of the stream going to waste is steady, the waste stream is passed directly through the thermal reservoir, entering at a high temperature end and leaving at a low temperature end. In the case of an open thermal reservoir, the top would be the high temperature end and the bottom the cold end. Thermal stratification is used to slow mixing in the tank. Mixing is undesirable, because available energy is lost when high temperature regions are allowed to mix or transfer heat with low temperature regions. Tubes in this instance can pass through the thermal reservoir from top to bottom. One of the problems with these prior art devices is that it is not generally convenient to pump waste water to the top of a reservoir. The use of short thermal reservoir is undesirable because thermal stratification is not as effective in blocking heat transfer.
Another problem is encountered with prior art devices where the waste stream is variable in temperature. In this instance cold as well as hot rinse water is sent to waste. Cold fluid entering the hot end of the reservoir cools it and disturbs the thermal stratification. In addition when the hot end of the reservoir is cooled, the discharge water from the reservoir is no longer hot and the heat load on an after-heater is increased.
The present invention permits a waste stream exchanger to be positioned at a low level and at a short and somewhat remote location. The present invention permits a waste stream to have a variable temperature without degrading the desirable high temperature strata in a thermal reservoir. In addition when high temperature waste becomes available, the present device automatically charges the highest temperature strata first, thus conserving the greatest amount of available energy.